Our main objective is to produce and detect chromosomal inversions and other specific chromosomal aberrations in mice by exposing sperm and spermatids to mutagens. We shall identify the genetic and chromosomal location of each inversion and maintain them either in homozygous condition or, if homozygous lethal, in balanced condition with a genetically marked normal homologue. Our objective is to use these inversions as tools for mammalian studies in mutagenesis, cytogenetics, linkage, meiotic and mitotic recombination, and dominant effects of recessive lethals and detrimentals. Nineteen of the inversions produced are being studied and used to assess the incidence of induced recessive lethals, to maintain and study three induced lethals in chromosome 1, to determine the effects on fitness and reproductive performance of the inversions or combinations of them, to assess the frequency of somatic crossing-over, to ascertain the nature of chromosomal break points and, thereby, to map genes cytologically.